Synthesis, structural characterization, and catalytic behaviour in Heck coupling of palladium(ii) complexes containing pyrimidine-functionalized N-heterocyclic carbenes

Abstract

[Pd(L1)₂(CH₃CN)](PF₆)₂ (L1 = 1-n-butyl-3-(2-pyrimidyl)imidazolylidene, 3) and [Pd(L2)₂](PF₆)₂ (L2 = 1-(2-picolyl)-3-(2-pyrimidyl)imidazolylidene 4), prepared viacarbene transfer reactions of [Ag(L1)₂]PF₆ (1) and [Ag₂(L2)₂](PF₆)₂ (2) with palladium salts, respectively, have been fully characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis. The X-ray crystal structures of complexes 1–4 are reported. Complex 3 is an unusual pentacoordinated palladium complex, in which the palladium is coordinated by two imidazolylidene, two pyridine, and one acetonitrile molecule in a square-pyramidal geometry. The apical position is occupied by a pyrimidine nitrogen atom with a relatively long Pd–N distance (2.762(6) Å). Complex 4 is a typical square-planar palladium complex with palladium surrounded by two pairs of cis-arranged pyridine and imidazolylidene ligands. The complexes exhibit good catalytic activities in the Heck coupling reaction of aryl bromides and activated aryl chlorides under mild conditions.